1. Field of the Invention
The present invention relates to an axial compressor that sprays a droplet in suction gas, and a gas turbine provided with the axial compressor.
2. Description of the Related Art
A gas turbine generally includes an axial compressor for compressing air, a combustor for mixing a fuel with the compressed air generated by the axial compressor and burning the fuel and the compressed air, and a turbine that is rotationally driven by combustion gas generated by the combustor. Increase in air temperature (or increase in temperature of suction air of the axial compressor), as in summer, reduces the density of the suction air and output of the gas turbine. To deal with this, a method for spraying droplets such as water droplets in the suction air of the compressor has been proposed (refer to, for example, JP-2010-48213-A).
When droplets are sprayed in the suction air of the compressor, the droplets evaporate on an air intake side of the compressor thereby depriving the air of heat. Thus, the temperature of the suction air is reduced, and the density of the suction air is increased. This inlet air cooling effect can improve the output of the gas turbine. Moreover, if the rate of spraying droplets is increased to, for example, 2% or more of a mass flow rate of the suction air to introduce droplets into the compressor (or the droplets that are not gasified on the air intake side of the compressor are introduced into the compressor), the droplets evaporate in the compressor, thereby depriving the air of heat. Thus, the temperature of air in the compressor is reduced. This intermediate cooling effect causes a compression characteristic to be close to isothermal compression, and reduces compression work. This enables the efficiency of the gas turbine to be improved.
As described above, when the droplets are introduced into the compressor and evaporate, a stage load on a front stage side (specifically, blade loading on the upstream side with respect to an evaporation completion position at which the droplets completely evaporate) is reduced, and a stage load on a rear stage side (specifically, blade loading on the downstream side with respect to the evaporation completion position) increases. It is, therefore, preferable that the compressor be designed such that the stage load on the front stage side is relatively high in expectation of the reduction in the stage load and the stage load on the rear stage side is relatively low in expectation of the increase in the stage load, so that an operation is to be optimized when droplets are sprayed in the suction air of the compressor (or when the droplets are introduced into the compressor). If, however, the compressor is designed in the aforementioned manner, when droplets are not sprayed in the suction air of the compressor (or the droplets are not introduced into the compressor), for example, upon activation of the gas turbine, a high load is applied to the blade cascade arranged on the front stage side. Further, in general, a high load is easily applied to the blade cascade arranged on the upstream side of the compressor during a low-speed rotation in an initial state of the activation of the gas turbine. For highly loaded rotor blades, a separation region may become larger due to interference of leaking flow occurred in a space between the rotor blades and a casing with a shock wave generated between the rotor blades. This raises a problem about a stall.
As a measure to suppress the stall on the highly loaded rotor blades, a casing treatment that has an annular groove (rectangular slit) formed in an inner circumferential surface of the casing so as to be positioned around the rotor blades has been disclosed (refer to, for example, JP-2009-236069-A). The groove suppresses the interference of the leaking flow occurred in the space between the rotor blades and the casing with the shock wave generated between the rotor blades.